This invention pertains to ceramic matrix composites that comprise ceramic fibers coated with at least one binary coating comprised of boron nitride (BN) and silicon nitride (Si3N4) within ceramic matrix. The ceramic matrix is derived from curable preceramic polymers. The composites can be formed into complex shapes which have good oxidation resistance at high temperature, high flexural strength and are resistant to moisture.
It is well known that for ceramic matrix composites, interfacial bonding between the reinforcing fiber and matrix controls the mechanical properties of the composite. In many ceramic matrix composites reinforced with siliconoxycarbide fibers, carbon coatings on the fiber have been shown to control interfacial bonding between fiber and matrix to produce desired mechanical properties. It is possible to apply the carbon coating to the fiber before fabricating the composite, or to process the composite for short durations at approximately 1000xc2x0 C. under non-oxidizing conditions to produce a thin carbon layer on the fiber.
Unfortunately, the use of these ceramic matrix composites in high temperature ( greater than 500xc2x0 C.), oxidizing environments tends to degrade strength and strain tolerance. In some cases it has been shown that the use of BN coatings in place of the carbon coating between the fiber and the matrix substantially improves the oxidative stability of the ceramic matrix composite. For example, U.S. Pat. No. 4,642,271 to Rice discloses a ceramic fiber composite material comprised of boron nitride (BN) coated ceramic fibers embedded in a ceramic matrix. U.S. Pat. No. 5,198,302 discloses silicon nitride reinforcing fibers provided with a protective surface coating comprising a BN base layer and optionally an alumina overcoating. U.S. Pat. No. 5,354,602 to Stranford et al. discloses the use of BN coated fibers in a matrix of black glass ceramic. U.S. Pat. No. 5,707,471 to Petrak et al., discloses the fibers coated with carbon, boron nitride, silicon carbide, silicon nitride, aluminum nitride and combinations of these.
It has now been found that ceramic matrix composites, with polymer derived matrices, that use BN coated fibers are susceptible to moisture corrosion at low temperatures (xe2x89xa6100xc2x0 C.).
U.S. Pat. Nos. 5,580,643 and 5,202,059 disclose duplex coated ceramic filler materials wherein the filler material may be a fiber and the coatings are boron nitride (BN) and silicon carbide (SiC). However, this duplex coating does not provide the benefits of this invention, including the moisture resistance.
Kowbel et al. in xe2x80x9cA Chemical Vapor Deposition (CVD) BN-Si3N4 Interfacial Coating for Improved Oxidation Resistance of SiC-SiC Compositesxe2x80x9d, Journal of Materials Synthesis and Processing, Vol. 3, No. 2 (1995) pp. 121-131 disclose the use of a mixture of BN and Si3N4 to coat SiC fibers. However, as can be seen in FIG. 11, these composites have about the same flexural strength as a BN coated fiber.
It is an object of this to provide ceramic matrix composites which contain coated fibers wherein the coating comprises at least one binary layer comprised of boron nitride and silicon nitride.
This invention pertains to a ceramic matrix composite comprising a ceramic fiber coated with at least one binary coating comprised of boron nitride (BN) and silicon nitride (Si3N4). The ceramic matrix composites containing the coated fibers maintain flexural strength when exposed to moisture.